Introduce toHTMLFormControlElement, and use it.

Source/core/html/HTMLKeygenElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2010 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 100 matching lines @@
 }
 
 const AtomicString& HTMLKeygenElement::formControlType() const
 {
     DEFINE_STATIC_LOCAL(const AtomicString, keygen, ("keygen", AtomicString::ConstructFromLiteral));
     return keygen;
 }
 
 void HTMLKeygenElement::reset()
 {
-    static_cast<HTMLFormControlElement*>(shadowSelect())->reset();
+    shadowSelect()->reset();
 }
 
 bool HTMLKeygenElement::shouldSaveAndRestoreFormControlState() const
 {
     return false;
 }
 
 HTMLSelectElement* HTMLKeygenElement::shadowSelect() const
 {
     ShadowRoot* root = userAgentShadowRoot();
     return root ? toHTMLSelectElement(root->firstChild()) : 0;
 }
 
 } // namespace